The semiconductor fabrication technique of microlithography defines the high resolution circuitry in a semiconductor device by exposing a photoresist on a substrate to radiation. A photoresist composition (also referred to herein as “photoresist” or simply “resist”) typically comprises a polymeric matrix, a radiation-sensitive component, a casting solvent, and other performance-enhancing additives. In practice, the resist is spin-coated onto a silicon wafer to form a coating that is typically 30 to 500 nm in thickness. The film is then heated to remove residual solvents; this step is generally referred to as post-apply bake, or PAB. The film is then exposed pattern-wise to radiation and, optionally, heated (in a step referred to as post-exposure bake, or PEB) to induce a chemical transformation that renders the solubility of the exposed areas of the film to be different from the solubility of the unexposed areas. The radiation used for exposure has typically been ultraviolet light with wavelengths ranging from the near ultraviolet (UV) to the deep UV (DUV) and extreme UV (EUV), thus including wavelengths of, for example, 436, 365, 257, 248, 193 or 13.5 nanometers (nm). Shorter wavelengths are currently preferred because of the higher resolution provided. A beam of electrons or ions, also known as “E-beam radiation” or “ion beam radiation,” respectively, has also been used. After exposure, the resist film is developed with a solvent to generate the resist image on the wafer. The resist is classified as a positive-tone resist (PTR) or a negative-tone resist (NTR) depending on the tone of the final image that is created. In positive-tone imaging, the exposed area of the resist film is rendered more soluble in the developer than the unexposed area, while in negative-tone imaging, it is the unexposed areas that are more soluble in the developer than the exposed areas.
Resists used with UV, DUV, EUV, and E-beam radiation generally require the use of chemical amplification in order to increase resist sensitivity, such that an agent (e.g., an acidic agent, termed a “photoacid”) that is generated by the interaction of the exposure radiation with a photoacid generator (PAG) acts as a catalyst to effect multiple reactions in the polymer structure, rendering the resist polymer more or less soluble in the developer, depending on the type of resist.
While most resist processing has typically involved PTRs, there has been an increased interest in developing NTRs in order to improve imaging performance and/or the process window of the resist at a particular masking level. There is still a dearth of chemically amplified negative-tone resist compositions, particularly for E-beam and EUV applications. Most of the current negative resists are based on acid-catalyzed cross-linking of polar functional groups. Such negative resists have not achieved equivalent performance to that of chemically amplified positive resists due to the inherent shortcomings in the cross-linking mechanism. Cross-linked polymers tend to swell in the developer, leading to imperfect images, resulting in micro-bridging between features. This becomes a critical issue for E-beam and EUV applications in particular, where high resolution and low line edge roughness (LER) are requirements.
An alternative approach was followed to prepare a negative-tone resist based on a polarity change mechanism wherein a photo-generated acid catalyzes the elimination of a polar functionality to decrease the dissolution rate of the resist in an aqueous base developer. This approach is described in US Patent Publication No. 2013/0209922 A1 to Masunaga et al., in which a high resolution negative resist based on compositions incorporating an acid-removable hydroxyl group is described. The aforementioned resists provided high resolution and low LER, but when defectivity tested, exhibited significant numbers of “blob-like” defects ranging in size from about 1 mm to about 5 mm in the unexposed regions of the wafer or mask substrate. The term “blob-like” defect refers to a streak, chain, or concentrated region of smaller defects, or to an agglomeration of smaller defects. Various process modifications were attempted to address the problem, but none sufficiently reduced the number or size of the defects. Lower molecular weight polymers, higher molecular weight polymers, and different polarity switching units were also tried, without success.
PAG-bound polymers have been reported in positive resist compositions in order to improve the lithographic performance; see, e.g., U.S. Pat. No. 7,812,105 to Nagai et al. A negative resist composition based on cross-linking chemistry using a PAG-bound polymer has also been reported, in U.S. Patent Publication No. 2012/0219888 A1.